A technique for control to be performed between two control units for controlling image formation is disclosed in, for example, Japanese Patent Application Laid-Open No. 2005-249808. According to the description of Japanese Patent Application Laid-Open No. 2005-249808, restart control means respectively for two control units (control means) are provided, so as to restart a second control unit (a sub control unit) by second restart control means for the second control unit when a first control unit (a main control unit) has run away. Specifically, when the main control unit has run away, the sub control unit is restarted, so as to prevent the sub control unit from being affected by the main control unit having run away.